The present invention relates to methods of producing quantum computer chips and the chips produced, and more particularly to methods of producing quantum computer chips including post-measurement qubit frequency modification of the chips produced.
When fixed-frequency transmons are fabricated, their actual frequency naturally deviates somewhat from the targeted fixed frequency. Consequently, for a chip that has a plurality of fixed frequency qubits, there will be an innate frequency spread for the population of qubits that is too high to reliably allow for the operation of cross resonance gates between every pair of qubits. The frequency spread results in “frequency collisions,” in which an undesired ZZ-interaction between a pair of qubits is too large perform a cross resonance gate, or else causes slow cross resonance gates due to the large detuning between a pair of qubits. Post-measurement annealing and lithographic modification can modify the qubit frequency by changing the Josephson junction inductance or capacitance, for example, by removing dielectric. However, post-measurement frequency modification often requires access to qubits on the qubit chip after the qubit chip is assembled on a substrate with readout circuits (or parts thereof). As quantum processors move to high density, flip chip-to-chip bump bonding becomes necessary. The qubit chip may be flip chip bonded to an interposer chip to perform measurements on the qubit chip, and to form a quantum-computer chip. However, currently, once the qubit chip is flip chip bonded to the interposer chip, it is no longer possible to access the qubit chip to perform frequency modification.